As a result of combustion process, two types of ash are formed in fluidized bed boilers: bottom ash and fly ash. The bottom ash is normally removed through the grate of the furnace from below, at a temperature of 600 to 900° C. The fly ash entrained in flue gases from the furnace is collected after all the heat surfaces at a temperature of 130 to 250° C. by means of an electrostatic precipitator or bag filter. Both bottom ash and fly ash can be recycled to the furnace.
The purpose of the incineration of waste is to reduce its volume for the final disposal and to recover its energy content. It can be estimated that a saving of at least 90% in the volume can be achieved by the incineration of landfill waste, resulting in ash that is more hygienic than the landfill waste. Waste materials produced by the incineration of waste include flue gases and solid ash. As in combustion processes in general, much attention has been paid to the control and processing of flue gases, to remove impurities. Also, attempts have been made to recover the ashes as well as possible before the final disposal.
In waste incineration, the use of fluidized bed techniques is a useful solution, but its competitive strength in comparison with the grate technique is weakened because of the larger amount of fly ash. The proportion of bottom ash, which is less problematic, is of the order of 20 to 30% of the total ash flow in the fluidized bed technique, whereas in the grate technique it is about 60 to 90%. This causes significant extra costs in the operational economy of a fluidized bed boiler plant.
In particular, the use of the fluidized bed technology in waste incineration is hindered by the waste fees that have been set considerably higher for fly ash than for bottom ash. The higher waste fees are due to the toxicity of the fly ash, because it contains heavy metals and dioxins. Attempts have been made to reduce the toxicity of the fly ash by further processing, which is expensive. A common method is to treat the ash chemically or to immobilize the components contained in the ash in a form in which their leaching from the ash, for example to the soil of the disposal site, is prevented. For example, U.S. Pat. No. 4,977,837 discloses a method, in which fly ash is separated from flue gas cooled down to about 315° C. (600 F) and is mixed with glass cullet that may originate from waste glass, and the mixtured is melted in a special vitrification oven that is heated to about 650° C. to melt the glass. The result is vitrified ash that can be used as landfill, road bed foundation, or construction material.
Disadvantages of the chemical processing methods include the extra investments and chemical costs required by them, and furthermore, if they are based on the washing of ash, the treatment of the solutions resulting from them. In the immobilization, in turn, even though the leaching can be prevented, the detrimental elements still remain encapsulated in the waste, which is always a factor of insecurity in long-term final disposal. On the other hand, if, for example, dioxins in the ash are to be decomposed, high temperatures must be used in the processing of the ash, which is energy consuming.
U.S. Pat. No. 5,086,715 discloses a method for preventing the de novo synthesis of dioxins and furans in flue gases by a so-called shock cooling step, in which the flue gases are quickly cooled by water from 450° C. to 250° C. before the step of separating the solids. In other words, this requires the supply of a particular cooling agent (water) by direct injection into the flue gas flow. The energy content corresponding to this temperature reduction is not recovered because of the cooling method.
German application publication DE 3733831 presents a coal-burning boiler coupled after a waste-incinerating fluidized bed boiler. The flue gases from the fluidized bed boiler are fed into the combustion zone of the coal-burning boiler, to destroy the dioxins contained in them at a high temperature preferably in the range of 1000 to 1200° C. After this, the solids are separated from the joint flue gases of the coal boiler and the waste boiler by an electrostatic precipitator and are cooled by a heat exchanger. It is obvious that waste incineration plants do not always have two boilers available, to be arranged one after the other in relation to the flue gas flow and one of them to be used for burning a more valuable fuel with better fuel economy, such as coal.
Attempts may be made to reduce the quantity of the fly ash to be discharged by recycling the fly ash separated by the filter into the furnace. In spite of this, the proportion of flue ash with a high waste fee remains high.
Finnish patent FI 110025 presents a method in which a particle-gas suspension from a fluidized bed reactor is divided in a cyclone separator into two fractions, of which the coarser fraction is removed from the bottom of the separator and recycled to the furnace, and the finer fraction is discharged via a central tube of the cyclone separator. This finer fraction, in turn, is divided into two fractions, of which the coarser part is separated by a particular vortex extractor and is recycled to the furnace, whereas the finer part is discharged from the central tube with the flue gas flow, and it can be separated by other methods. By this solution, it is possible to raise the degree of separation by the cyclone separator and to recycle the particles that would otherwise be discharged with the flue gas flow, by a simple arrangement into the process. Consequently, also this publication presents a technical solution based on the recycling of fly ash.